remove dependencies to openbsc/osmoggsn internals

...and link against libosmogb (part of libosmocore.git)
diff --git a/src/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index 8d391f2..9084d84 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -279,3 +279,8 @@
 	}
 	return rc;
 }
+
+int bssgp_prim_cb(struct osmo_prim_hdr *oph, void *ctx)
+{
+	return 0;
+}